channel.c: ELOG already prefixes each line with "error @ ..."

ex_cmds2.c: remove obsolete comment per
  https://github.com/neovim/neovim/commit/8a5a8dbf0f8dfec0b0494a0215891469753961a1#commitcomment-8862443

misc. minor fixes

mark +clipboard feature

main.c: usage(): edit for concision, layout